1. Field of the Invention
The present invention relates to a digital double demodulator.
The invention relates to signals transporting an information on a frequency, amplitude or phase-modulated carrier wave. Provided that there is a spectral compatibility of the modulations, it also applies to signals modulated both in amplitude and in frequency, or in amplitude and in phase, or to signals with double modulation of quadrature amplitude and suppressed carrier.
Thus, the invention particularly applies to television signals, especially to the demodulation of the chrominance signal of composite signals which, in the case of the SECAM system, is modulated in frequency and which, in the case of the PAL and NTSC systems, has undergone a double amplitude in quadrature modulation. It also applies to the demodulation of frequency-modulated signals used in video-recorders. It further applies to the demodulation of signals used in radiophony and data transmission, no matter whether the modulations have several phase, frequency or amplitude states. The performances obtained also permit applications in metrology, inter alia for instantaneous or non-instantaneous frequency measurements.
2. Discussion of the Background
The most widely known prior methods of frequency demodulation are of the analog type. Ratio or phase discriminators are of the earliest types. Systems are also known, whose operation is based on a period measurement, either by counting or by measuring the capacity charge. Although these methods are relatively adequate, they suffer from the disadvantage of leading to systems requiring sometimes difficult settings, of only accepting signals with a relatively limited band width and of being inappropriate for the digital processing of demodulated signals.
Digital methods dedicated to this type of demodulation have recently appeared, but hitherto none of them has led to completely satisfactory results. The various known algorithms are as follows:
1. Approximation Algorithms
The so-called "arc tangent" algorithm, in which the signal is sampled close to four times the carrier frequency and the position of the rotary vector associated therewith is estimated by calculating the arc tangent of the ratio of two samples approximately in quadrature. This not very accurate method suffers from the disadvantage of being closely linked with the sampling frequency used and of only permitting demodulation in a reduced frequency band. Such a method is e.g. described in FR-A-2 488 755.
Algorithm for estimating the zero passage points in which, on the basis of two samples of the modulated signal of opposite signs, using an interpolation method an estimate is made of the time at which the carrier is suppressed and as a result of this the period of the signal is calculated. This method requires a very high sampling frequency and is not very accurate, because the result is highly dependent on the quantification of the signal to be demodulated.
2. Division Algorithm
This method is based on the division of weighted sums of successive samples of the signal. Although theoretically accurate, calculations of this type cause problems of accuracy and stability for low values of the divider. It is therefore necessary to eliminate the results obtained when the divider is too small and to substitute them by results estimated on the basis of the preceding results or those immediately following the sought value. This leads to an inaccurate demodulated signal and which is in general very noisy. Such a method is e.g. described in FR-A-2 469 824, EP-A-107884, EP-A-68571 and EP-A-68579
3. Multiplication Algorithm
This consists of the transposition of a method used in analog form, according to which a product is formed between the modulated signal and said same signal passed through a delay line. The modulating signal is extracted from this result by low-pass filtering. In analog where, before demodulation, the signal passes through a limiter, said method gives good results. In digital, the limitation function has no sense, so that the signal demodulation according to this method is disturbed by all the amplitude distortions of the modulated signal, so that the result can again be very noisy.
With regards to the amplitude demodulation, the most widely known methods are also performed with analog circuits. The most sophisticated of them are so-called synchronous demodulations for which the carrier frequency, whether transmitted or not, is restored at the demodulator. The product between this wave and the signal to be demodulated permits, following low-pass filtering, the extraction of the modulating signal.
The known digital methods are a simple transposition of these analog methods. They lead to good results, but require relatively complicated low-pass filters.